1. Field of the Invention
The present invention relates generally to semiconductor lasers. In particular, the invention relates to a diode laser.
2. Technical Background
Semiconductor diode lasers emitting in the infrared portions of the spectrum have been sufficiently developed that they are widely used in a variety of applications. In one application, a high-power laser emitting, for example, at around 980 nm, optically pumps an erbium-doped fiber amplifier (EDFA). It is known that such lasers can be formed from layers of, for example, GaInAs or AlGaAs or related materials grown on a GaAs substrate.
For a typical edge-emitting laser, a p-n junction is formed by differential doping of the semiconductor layers, and electrical contacts formed above and below the junction to provide the electrical power to forward bias the laser and thereby to electrically pump it. Advanced structures include one or more very thin undoped active semiconductor regions formed into quantum wells between much thicker p-doped and n-doped semiconductor layers acting both as optical cladding layers and forming a vertical p-n diode structure. Multiple quantum wells are electronically isolated by barrier layers. The composition and thickness of the quantum wells allow precise tuning of the emission wavelength, and hence the lasing wavelength. A horizontally extending waveguide for the lasing radiation is formed by vertical and horizontal optical confinement structures. Mirrors, typically formed on the edges of the opto-electronic chip, define the ends of the laser cavity. The vertical optical confinement structure is usually closely associated with the p-n junction by appropriate compositional profiles. The horizontal confinement can be achieved by several structures, the two which will be discussed here being the etched ridge and the buried ridge.
In the etched ridge structure, the upper semiconductor cladding layer, which for example is a p-type layer, is selectively etched down close to but as far as the active layer to form a ridge in the upper cladding layer having a width of 2 to 5 .mu.m, but leaving a thin portion of the upper cladding layer. The sides of the ridge are either exposed to ambient or covered with a material of low dielectric constant to thus provide a single-mode waveguiding structure. The ridge height is usually comparable to its width, but it effectively and horizontally confines the light to a region mostly below the ridge. One electrical contact is made to the top of the ridge while typically the bottom of the substrate is electrically grounded for the other contact. The ridge provides the additional function of current confinement to guide the biasing current to a narrow horizontal extent of the underlying active layer corresponding to the ridge width so that biasing current is not wasted in areas outside of the waveguide.
The etched-ridge structure, however, suffers several problems when applied to a high power laser. The narrow width of the ridge and its upward projection from the substrate increases the series electrical resistance for the biasing current and also increases the thermal impedance for heat generated in the ridge. Furthermore, etching of the ridge is usually performed by diffusion-limited wet chemical etching resulting in a flared ridge, but the high power performance depends critically on the etching profile of the ridge. As a result, etched ridge lasers suffer poor reproducibility.
The buried ridge structure avoids the projecting etched ridge and its problems. Instead, the growth of the upper semiconductor cladding layer, for example, of p.sup.+ -doped AlGaAs, is divided into two portions. After a bottom portion of p.sup.+ -doped Al.sub.c Ga.sub.1-c As, is deposited, a barrier or confinement layer of, for example, n.sup.+ -doped Al.sub.b Ga.sub.1-b As of higher aluminum content (b&gt;c) is grown on the lower portion of the Al.sub.c Ga.sub.1-c As, and a hole is patterned and etched down to the underlying p.sup.+ -doped Al.sub.c Ga.sub.1-c As layer. An upper portion of the p.sup.+ -doped Al.sub.c Ga.sub.1-c As cladding layer is then regrown both in the hole over the exposed p.sup.+ -doped Al.sub.c Ga.sub.1-c As and over the top of the oppositely doped Al.sub.b Ga.sub.1-b As barrier layer. The opposite doping of the barrier layer confines the biasing current to the hole through the barrier layer. The upper portion of the upper cladding layer within the hole operates as a ridge extending upwardly from the lower portion. The thickness of the lower portion of the upper cladding layer is less than that needed to vertically confine the light, but the additional thickness of the ridge does confine it, both vertically and horizontally.
Typically, an n.sup.+ -doped Al.sub.p Ga.sub.1-p As protective layer of lower aluminum content (p&lt;b) is grown on the Al.sub.b Ga.sub.1-b As barrier layer prior to the hole etch in order to prevent the aluminum-rich barrier layer from being oxidized prior to regrowth. However, the protective layer does not protect the sidewall of the barrier layer after the hole etching and prior to the regrowth. Oxidation of the sidewall can lead to poor laser reliability. In general, to obtain a highly reliable laser, any aluminum-rich layer should be avoided for two reasons. Such a layer is subject to a higher degree of oxidation at any cleaved facet. Furthermore, it introduces tensile lattice strain relative to the aluminum-lean layers since the lattice constant of AlAs is less than that of GaAs.
It is thus desired to obtain a buried ridge laser that does not use an aluminum-containing barrier layer or other aluminum-containing layer exposed prior to regrowth. It is also desired to obtain a buried ridge laser that does not use an aluminum-rich layer that would be exposed during cleaving.